It is customary in the xerographic art to form an electrostatic latent image on a photoreceptor drum or plate comprising a charge conductive backing such as, for example, a metallic or metal coating surface having a photoconductive insulating layer applied thereto in good charge blocking contact. A suitable device for this purpose comprises, for example, an aluminum plate having a thin layer of vitreous selenium and an aluminum oxide and/or polymeric interlayer. Such a plate is characterized by being capable of excepting a suitable electrostatic charge and of quickly and selectively dissipating a substantial part of the charge where light is exposed. In general, such photoreceptors are sensitive to light in the blue-green spectral range.
While selenium containing photoconductive elements are usefully employed in commercial xerography, there is room for substantial improvement in photoconductive properties such as the range of spectral response, heat and charge stability, etc. These can be improved by the addition of various photoconductive alloys, alloying elements or other types of additives (ref. U.S. Pat. Nos. 2,803,542 and 2,822,300). For example, the addition of various amounts of arsenic can result in a broader range of spectral sensitivity and improve overall photographic speed and stability. Suitable alloys or homogeneous mixtures of elemental selenium with other metals suitable for this purpose can also be incorporated into the usual photoconductive material by conventional vacuum evaporation techniques. For example, additional inorganic coating materials can be placed in open or shuttered crucibles in a vacuum during an initial coating step. The xerographic substrate upon which the photoconductive material is to be deposited is conveniently placed above or in some other convenient location with respect to the potential coating vapor source. After the container has been evacuated to a suitable pressure (about 5 .times. 10.sup.-.sup.5 Torr), the vessel containing photoconductive material and/or additive is then heated by suitable means known to the art such as by electric resistance heating elements to promote vaporization of the material. At least some of the vaporized material then condenses on the relatively cool substrates; such a deposition process normally requires a period of about 15-60 minutes, depending upon the amount of substrate surface to be coated and the desired thickness of coating material.
From time to time it is also found desirable to apply profile concentrations of one or more photoconductive components or separate layers of different photoconductive materials to obtain a particular desired spectrum of characteristics. In such case, the respective photoconductive materials or alloys are most conveniently applied to substrates or bases by coevaporation techniques, in which predetermined amounts of the respective photoconductive materials or alloys are placed in separate crucibles or in subdivided crucibles and exposed or heated in a predetermined sequence under vacuum. One very useful modification for this purpose involves coating in the presence one or a plurality of elongated crucibles heated by electrical heating elements or by other conventional means, the crucibles being subdivided into a plurality of compartments or bins, each capable or carrying different amounts and kinds of coating materials depending upon the desired final concentration. Another useful modification involves the formation of one or more trains of small crucibles temporarily connected to each other and containing various photoconductive materials. Both arrangements are found to be very useful in coating a plurality of substrates simultaneously with a plurality of components.
Unfortunately, however, the use of such crucibles separated by baffles or end walls also presents serious technical problems insofar as it is difficult to control surface irregularities and achieve consistency during batch coating. This is found to be due largely to variation in the geometric relation of substrates to crucible bins and particularly attributable to the presence of crucible end walls or baffles. Such coating irregularities, in turn, usually cause unacceptable variations in electronic properties both between and within the individual photoreceptors being batch produced.
It is an object of the present invention to develop a method and equipment for efficiently and evenly batch coating one or more receiving surfaces or prepared substrates with one or more coating materials or components thereof.
It is also an object of the present invention to minimize or avoid irregularities when batch coating one or more inorganic photoconductive materials onto prepared xerographic substrates in a vacuum coater.
A still further object relates to obtaining an improved method for improvement quality of batch coated xerographic photoreceptors containing one or more photoconductive components.